Magnetic memory cells with high write current and read stability

ABSTRACT

Memory cells and methods of forming thereof are disclosed. The memory cell includes a substrate and first and second select transistors. The first select transistor serves as a write selector and the second select transistor serves as a read selector. The gate of first select transistor is coupled to a write wordline (WL_w) and the gate of the second select transistor is coupled to a read/write wordline (WL_r/w). The source regions of the first and second select transistors are coupled to a source line (SL). A body well is disposed in the substrate. The body well serves as a body of the first and second select transistors. A back bias is applied to the body of the select transistors. A storage element which includes a magnetic tunnel junction (MTJ) element is coupled with a bitline (BL) and the first and the second select transistors.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application Ser.No. 62/110,606, filed on Feb. 2, 2015, which is herein incorporated byreference in its entirety for all purposes.

BACKGROUND

A magnetic memory cell stores information by changing electricalresistance of a magnetic tunnel junction (MTJ) element. The MTJ elementtypically includes a fixed (pinned) magnetic layer and a free magneticlayer. The fixed (pinned) magnetic layer and the free magnetic layer arelaminated such that a tunnel barrier film is formed between the twolayers. Magnetic orientation of the free layer flips by a direction oran opposite direction of electric currents exceeding a critical selectcurrent. The electrical resistance of the MTJ element changescorresponding to that of the magnetic orientation of the free layerrelating to the fixed (pinned) magnetic layer, which may be in either aparallel state or an anti-parallel state.

Magnetic memory cells require high write current to improve write speedfor programming. However, high write current also results in a higherread current. High read current increases read disturb. For example, ifthe read current exceeds the critical switching current of MTJ (I_(C0)),the read current could cause the free layer to switch directions,resulting in a read error.

From the foregoing discussion, it is desirable to provide magneticmemory cells with high write speed and read stability.

SUMMARY

Embodiments of the present disclosure generally relate to memory devicesand methods for forming a memory device. In one embodiment, a memorycell is disclosed. The memory cell includes a substrate defined with amemory cell region and a cell selector unit disposed on the substrate.The cell selector unit includes a first select transistor having a firstgate coupled to a first word line (WL) and first and second source/drain(S/D) regions, and a second select transistor having a second gatecoupled to a second WL and first and second S/D regions. The firstselect transistor serves as a write selector and the second selecttransistor serves as a read selector. The first WL is a write wordline(WL_w) and the second WL is a read/write wordline (WL_r/w). The secondS/D regions of the first and second select transistors are coupled to asource line (SL). A body well is disposed in the substrate. The bodywell serves as a body of the first and second select transistors. A backbias is applied to the body of the select transistors. A storage elementwhich includes a magnetic tunnel junction (MTJ) element is coupled witha bitline (BL) and the first and the second select transistors.

In another embodiment, a memory cell is presented. The memory cellincludes a substrate defined with a memory cell region and a cellselector unit disposed on the substrate. The cell selector unit includesa first select transistor having a first gate coupled to a first wordline (WL) and first and second source/drain (S/D) regions, and a secondselect transistor having a second gate coupled to a second WL and firstand second S/D regions. The first select transistor serves as a writeselector and the second select transistor serves as a read selector. Thefirst select transistor is a low threshold voltage (LVT) transistor andthe second select transistor is a high threshold voltage (HVT) orregular threshold voltage transistor (RVT). The first WL is a writewordline (WL_w) and the second WL is a read/write wordline (WL_r/w). Thesecond S/D regions of the first and second select transistors arecoupled to a source line (SL). A storage element which includes amagnetic tunnel junction (MTJ) element is coupled with a bitline (BL)and the first and the second select transistors.

In yet another embodiment, a method of operating a memory cell isdisclosed. The method includes providing a memory cell as describedabove. The method further includes performing a read operation or writeoperation with the memory cell. When a write operation is performed, theWL_w and the WL_r/w are activated such that a write path is formedthrough both the first and second select transistors. When a readoperation is performed, the WL_w is inactive and the WL_r/w is activatedsuch that a read path is formed through the second select transistor.

In another embodiment, a method of forming a memory cell is presented.The method includes providing a substrate defined with a memory cellregion and forming a cell selector unit on the substrate. A first selecttransistor having a first gate coupled to a first word line (WL) andfirst and second source/drain (S/D) regions is formed. A second selecttransistor having a second gate coupled to a second WL and first andsecond S/D regions is formed. The first select transistor serves as awrite selector and the second select transistor serves as a readselector. The first WL is a write wordline (WL_w) and the second WL is aread/write wordline (WL_r/w). The second S/D regions of the first andsecond select transistors are coupled to a source line (SL). A body wellis formed in the substrate. The body well serves as a body of the firstand second select transistors. A back bias is applied to the body of theselect transistors. A storage element which includes a magnetic tunneljunction (MTJ) element is formed and is coupled with a bitline (BL) andthe first and the second selectors.

These and other advantages and features of the embodiments hereindisclosed, will become apparent through reference to the followingdescription and the accompanying drawings. Furthermore, it is to beunderstood that the features of the various embodiments described hereinare not mutually exclusive and can exist in various combinations andpermutations.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form part ofthe specification in which like numerals designate like parts,illustrate preferred embodiments of the present disclosure and, togetherwith the description, serve to explain the principles of variousembodiments of the present disclosure.

FIGS. 1a-1b show diagrams of parallel and anti-parallel states ofmagnetic tunneling junction (MTJ) elements;

FIGS. 2a-2b show schematic diagrams of various embodiments of memorycells;

FIGS. 3a-3b illustrate various embodiments of a memory array;

FIGS. 4a-4b show top and cross-sectional views of an embodiment of amemory cell;

FIGS. 5a-5b show top and cross-sectional views of another embodiment ofa memory cell;

FIGS. 6a-6m show cross-sectional views of an embodiment of a process forforming a device with a memory cell; and

FIGS. 7a-7d show cross-sectional views of another embodiment of aprocess for forming a device with a memory cell.

DETAILED DESCRIPTION

Embodiments of the present disclosure generally relate to memory cells.In one embodiment, the memory cells are magnetic resistive memory cells,such as magnetic random access memory (MRAM). Magnetic resistive memorycells include magnetic tunneling junction (MTJ) elements. Other suitabletypes of resistive memory cells may also be useful. The memory cells areconfigured to produce fast write time with read stability. The memorycells can be incorporated into or used with, for example, electronicproducts such as mobile phones, smart cards, mass storage, enterprisestorage and industrial and automotive products.

FIG. 1a shows a simplified cross-sectional view of an embodiment of aMTJ element 110 a. As shown, the magnetization or magnetic orientationsof the magnetic element are configured to be in a perpendicular orvertical direction. Perpendicular direction, for example, refers to adirection which is perpendicular to a substrate surface. A MTJ elementincludes a magnetically fixed (pinned) layer 126, a tunneling barrierlayer 127 and a magnetically free layer 128. The magnetic orientation ofthe fixed layer is fixed while the magnetic orientation of the freelayer may be programmed to be in a first or same direction as the fixedlayer or in a second or opposite direction as the fixed layer. Forexample, as shown by structure 111, the magnetic direction of the freelayer is programmed to be in a first or parallel direction as the fixedlayer. The corresponding MTJ electrical resistance between the freelayer 128 and the fixed layer 126 is denoted as R_(P). Structure 112illustrates that the magnetization of the free layer is programmed to bein a second or an anti-parallel direction as the fixed layer. Thecorresponding MTJ electrical resistance between the free layer 128 andthe fixed layer 126 is denoted as R_(AP). The resistance R_(AP) ishigher than the resistance R_(P). By way of example, the free layer andthe fixed or reference layer may be CoFeB or CoFe based compositematerial and the tunneling barrier layer may be MgO or Al₂O₃. The MTJstack may include other layers as well. Other suitable materials for theMTJ stack may also be useful.

FIG. 1b shows magnetic orientations in the magnetic layers of a MTJelement 110 b in a horizontal direction. Horizontal direction, forexample, refers to a direction which is parallel to the surface of asubstrate surface or in-plane with the magnetic layers. As shown in FIG.1b , the MTJ element 110 b includes a fixed (pinned) layer 126, abarrier layer 127 and a free layer 128. When the MTJ element 110 b is ina magnetic anti-parallel state, magnetic orientations of the free layer128 and the fixed layer 126 are in opposite directions, as shown bystructure 111. The corresponding MTJ electrical resistance between thefree layer 128 and the fixed layer 126 is denoted as R_(AP). When theMTJ element 110 b is in a magnetic parallel state, magnetic orientationsof the free layer 128 and the fixed layer 126 are in the same directionas shown by structure 112. The corresponding MTJ electrical resistancebetween the free layer 128 and the fixed layer 126 is denoted as R_(P).

As shown in FIGS. 1a-1b , the MTJ stacks are bottom pinned stacks. Forexample, in a bottom pinned stack, the fixed layer is disposed below thefree layer. In other embodiments, the MTJ stacks may be top pinned MTJstacks. For example, the fixed layer may be disposed above the freelayer.

FIG. 2a shows a schematic diagram of an embodiment of a memory cell 200.The memory cell is a non-volatile memory cell. For example, the memorycell may be a magnetic memory cell. In one embodiment, the memory cellis a Spin Torque Transfer-Magnetic Random Access Memory (STT-MRAM) cell.Other suitable types of memory cells may also be useful. The memory cellincludes a storage unit 210 and a cell selector unit 240. The storageunit is coupled to the cell selector unit. The storage unit 210, in oneembodiment, includes a storage element 220. The storage element, forexample, may be a resistive storage element. In one embodiment, thestorage element is a magnetic resistive memory storage element, such asa MTJ storage element. The MTJ element may include MTJ elements such asthose described in FIGS. 1a-1b . Other suitable types of MTJ orresistive storage elements may also be useful.

In one embodiment, the MTJ element is a bottom pinned perpendicular MTJelement. Other types of MTJ elements, such as horizontal or top pinnedMTJ elements may also be useful. The top of the MTJ stack is coupled toa bitline BL while the bottom of the MTJ stack is coupled to theselector unit. In some cases, the MTJ element may include top and/orbottom electrodes (not shown). For example, the top electrode isdisposed over the top of the MTJ element and the bottom electrode isdisposed below the MTJ stack. The electrodes may be Ta electrodes. Othertypes of electrodes may also be useful. In such cases, the top electrodeis coupled to a bitline BL of the memory cell while the bottom electrodeis coupled to the selector unit.

As for the cell selector unit 240, it includes first and secondselectors or selector subunits 241 a-241 b. The first selector subunit241 a, for example, may be a write selector and the second selectorsubunit 241 b may be a read selector. In one embodiment, the secondselector subunit 241 b is a read/write selector unit. A selector subunitincludes a select transistor. For example, the first selector subunitincludes a first select transistor and the second selector subunitincludes a second select transistor. A select transistor of a subunitis, for example, a metal oxide semiconductor (MOS) transistor. In oneembodiment, the select transistor is an n-type MOS transistor. Providinga p-type MOS transistor as a select transistor may also be useful.

A select transistor, as shown, includes first and second source/drain(S/D) terminals 245 and 246, a gate or control terminal 244 and a body208. The body of the transistor, for example, is a doped well having theopposite polarity type as the transistor type. For example, an n-typetransistor has a p-type body. In one embodiment, a back bias B_B isapplied to the body of the select transistor. The S/D terminals 245 and246 are formed by, for example, heavily doped regions having the samepolarity type dopants as the transistor type. For example, the S/Dterminals are heavily n-doped regions for an n-type transistor. Thefirst S/D terminal 245 may be referred to as a drain terminal and thesecond S/D terminal 246 may be referred to as a source terminal. Asshown, the drain terminals of the select transistors are commonlycoupled to the MTJ element. For example, the drain terminals are coupledto the bottom electrode of the MTJ element. The source terminals of theselect transistors are commonly coupled to a source line SL of thememory cell.

In one embodiment, a write wordline (WL_w) is coupled to the gateterminal of the write selector 241 a and a read wordline is coupled tothe gate terminal of the read selector 241 b. During a write operation,the WL_w is activated, causing the write selector to couple the storageunit to the SL, creating the write path through the first selecttransistor. During a read operation, the read wordline is activatedwhile WL_w is inactive. This forms a read path through the second selecttransistor to the SL.

In one embodiment, the read wordline can be activated during a writeoperation as well. For example, the gate of the second transistor 241 bis coupled to a read/write wordline (WL_r/w). As described, the writepath is formed through both the first and second select transistors.Providing a write current by both transistors increases the writecurrent to program the MTJ element. Depending on whether the write is a“1” or a “0”, the write current is from the BL to the SL through bothtransistors or from SL to the BL through both transistors. As for theread path, it is formed only through the second transistor. This reducesthe read current to the MTJ stack, resulting in more stable readoperations. Furthermore, the back bias applied to the transistor bodyoffers various advantages. For example, a forward back bias (i.e.positive voltage for NFET) applied to the transistor body increases thewrite current and a reverse back bias (i.e. negative voltage for NFET)applied to the transistor body reduces the read current.

Table 1 below shows exemplary signals or bias conditions applied to thememory cell of FIG. 1a for read and write operations.

TABLE 1 signal (V) BL SL WL_w WL_r/w B_B operation Sel Unsel Sel UnselSel Unsel Sel Unsel Sel Unsel Read Med Low Low Low Low Low High Low NegNeg Write “1” High Low Low Low High Low High Low Pos Pos Write “0” LowLow High Low High Low High Low Pos PosThe voltage values for a logic medium (“Med”) is about 0.1-0.2V, a logic“High” is 1.0V and a logic “Low” is 0V. In the case of a back bias(B_B), a logic “Neg” is less than 0 to −2V while a logic “POS” isgreater than 0 to 0.6V for a silicon substrate or 0 to 2.0V for a fullydepleted silicon-on-insulator (FDSOI) substrate. The voltage values areexemplary and may change depending on, for example, the technology node.In addition, the voltage and logic values as presented based on a memorycell with n-type MOS transistors and for MTJ stack with a bottom-pinnedstructure, i.e. free layer or storage layer is disposed on top of thetunnel barrier. The values may change for other types of selecttransistors.

FIG. 2b shows a schematic diagram of another embodiments of a memorycell 200. The memory cell is a non-volatile memory cell, such as amagnetic memory cell. For example, the memory cell may be a STT-MRAMcell. The memory cell is similar to the memory cell described in FIG. 2a. Common elements may not be described or described in detail.

The memory cell includes a storage unit 210 and a cell selector unit240. The storage unit is coupled to the cell selector unit. The storageunit 210, in one embodiment, includes a storage element 220, such as aMTJ storage element. The top of the MTJ stack is coupled to a bitline BLwhile the bottom of the MTJ stack is coupled to the selector unit. TheMTJ stack may include top and/or bottom electrodes (not shown).

As for the cell selector unit 240, it includes first and secondselectors or selector subunits 241 a-241 b. The first selector 241 a,for example, may be a write selector and the second selector 241 b maybe a read or a read/write selector. A selector includes a selecttransistor, such as an n-type MOS transistor. Providing a p-type MOStransistor may also be useful.

A select transistor includes first and second source/drain (S/D)terminals 245 and 246, a gate or control terminal 244 and a body 208.The first S/D terminal 245 may be referred to as a drain terminal andthe second S/D terminal 246 may be referred to as a source terminal. Thedrain terminals of the select transistors are commonly coupled to theMTJ element; the source terminals of the select transistors are commonlycoupled to a source line SL of the memory cell.

In one embodiment, a WL_w is coupled to the gate terminal of the firsttransistor and a WL_r/w is coupled to the second transistor. In oneembodiment, the first transistor 241 a is a transistor with a lowthreshold voltage (LVT). For example, the LVT is about 0.1-0.3V. As forthe second transistor 241 b, it is a high threshold voltage (HVT)transistor or a regular voltage threshold (RVT) transistor. HVT or RVTis higher than LVT. For example, HVT may be about 0.4-0.6V and RVT maybe about 0.2-0.4V. Other suitable voltage values may also be usefuldepending on, for example, technology node. Providing a HVT or RVTtransistor for read operations decreases read current, improving readstability.

The operating biases for the memory cell is similar to that provided inTable 1 above, except that the bodies of the select transistors are notback biased. In some applications, the memory cell of FIG. 2b may beprovided with a back bias to the bodies of the select transistors. Insuch case, a forward back bias (i.e. positive voltage for NFET) appliedto the transistor body increases the write current and a reverse backbias (i.e. negative voltage for NFET) applied to the transistor bodyreduces the read current.

FIG. 3a shows a schematic diagram of an embodiment of a memory array300. The array includes a plurality of interconnected memory cells 200.The memory cells may be similar to the memory cell described in FIG. 2a. For example, the memory cells are MRAM cells, such as STT-MRAM cells.Other types of memory cells may also be useful. Common elements may notbe described or described in detail. In one embodiment, the memory cellsinclude select transistors having bodies which are back biased.

As shown, the array includes six memory cells arranged in a 3×2 array.For example, the array is arranged to form three rows and two columns ofmemory cells. Memory cells of a row are interconnected by WL_w/r, WL_w,and SL lines (WL_r/w0, WL_w0 and SL0; WL_r/w1, WL_w1 and SL1; orWL_r/w2, WL_w2, SL2) while memory cells of a column are interconnectedby a BL line (BL0; or BL1).

FIG. 3b shows a schematic diagram of another embodiment of a memoryarray 300. The array includes a plurality of interconnected memory cells200. The memory cells may be similar to the memory cell described inFIG. 2b . For example, the memory cells are MRAM cells, such as STT-MRAMcells. Other types of memory cells may also be useful. Common elementsmay not be described or described in detail. In one embodiment, thefirst select transistor is a LVT transistor and the second selecttransistor is a HVT transistor. Providing a memory cell with both backbiased transistor bodies as well as LVT as the first select transistorand HVT as the second select transistor may also be useful.

As shown, the array includes six memory cells arranged in a 3×2 array.For example, the array is arranged to form three rows and two columns ofmemory cells. Memory cells of a row are interconnected by WL_w/r, WL_w,SL lines (WL_r/w0, WL_w0 and SL0; WL_r/w1, WL_w1 and SL1; or WL_r/w2,WL_w2, SL2) while memory cells of a column are interconnected by a BLline (BL0; or BL1).

FIGS. 4a-4b show top and cross-sectional views of an embodiment of adevice 400. The device, as shown, includes a memory cell. The memorycell, for example, may be a NVM memory cell. The memory cell, in oneembodiment, is a magnetoresistive NVM cell, such as a STT-MRAM cell. Thememory cell, for example, is similar to that described in FIG. 2 a.Common elements may not be described or described in detail.

The memory cell is disposed on a substrate 405. For example, the memorycell is disposed in a cell region of the substrate. The cell region maybe part of an array region. For example, the array region may include aplurality of cell regions. The substrate may include other types ofdevice regions (not shown), such as high voltage (HV) as well as logicregions, including low voltage (LV) and intermediate voltage (IV) deviceregions. Other types of regions may also be provided.

The substrate, for example, is a semiconductor substrate, such as asilicon substrate. For example, the substrate may be a lightly dopedp-type substrate. Providing intrinsic or other types of dopedsubstrates, such as silicon germanium (SiGe), germanium (Ge), galliumarsenide (GaAs) or any other suitable semiconductor materials, may alsobe useful. In some embodiments, the substrate may be acrystalline-on-insulator (COI) substrate. A COI substrate includes asurface crystalline layer separated from a crystalline bulk by aninsulator layer. The insulator layer, for example, may be formed of adielectric insulating material. The insulator layer, for example,includes silicon oxide, which provides a buried oxide (BOX) layer. Othertypes of dielectric insulating materials may also be useful. The COIsubstrate, for example, is a silicon-on-insulator (SOI) substrate. Forexample, the surface and bulk crystalline layers are single crystallinesilicon. Other types of COI substrates may also be useful. It isunderstood that the surface and bulk layers need not be formed of thesame material.

Isolation regions 480 may be provided. In one embodiment, the isolationregions are shallow trench isolation (STI) regions. Other types ofisolation regions may also be useful. The isolation regions are providedto isolate device regions from other regions. In one embodiment,isolation regions may be provided to isolate memory cells from eachother. For example, a memory cell may be formed in a cell region whichis isolated from other cell regions by isolation regions. Other suitableconfigurations of isolation regions may also be useful.

A cell selector unit is provided in the cell region. The cell selectorunit includes first and second selectors 441 a-441 b. The firstselector, for example, may be a write selector and the second selectoris a read/write selector. The selectors, in one embodiment, includetransistors. For example, the transistors of the selectors are metaloxide semiconductor (MOS) transistors. In one embodiment, the MOStransistors are n-type MOS transistors. Other types of cell selectorsmay also be useful.

The cell region includes a device well or body well 408 which serves asa body of the transistors. For example, the cell region includes a bodyor device well for transistors of the selectors. The device well may bea continuous well for the entire memory array. The body well may bedoped with second polarity type dopants for first polarity type selecttransistors. The body well, in one embodiment, is a p-type doped wellfor n-type transistors. The device well may be lightly or intermediatelydoped with second polarity type dopants. For example, the device wellmay have a dopant concentration of about 1E16 to 1E19/cm³. Othersuitable dopant concentrations may also be useful.

A transistor, as shown, includes first and second source/drain (S/D)regions 445 and 446. A gate 444 is disposed on the substrate between theS/D regions. A gate, for example, may include a gate electrode over agate dielectric. The gate electrode may be polysilicon while the gatedielectric may be silicon oxide. Other suitable types of gate electrodeand gate dielectric materials may also be useful. For example, a gatemay be a metal gate with a metal gate electrode and a high k gatedielectric. A gate, for example, may be a gate conductor along a firstor word line (WL) direction. The gate conductor forms a common gate fora row of memory cells.

As for the S/D regions, they are heavily doped regions with firstpolarity type dopants. For example, the S/D regions are heavily dopedn-type regions for forming n-type transistors. Providing S/D regionswith p-type dopants may also be useful. The S/D regions may includelightly doped (LD) extension regions. The gate may include dielectricsidewall spacers 449. The spacers, for example, facilitate in formingthe lightly doped (LD) extension regions 447 and heavily doped S/Dregions. The LD extension regions, for example, are lightly dopedregions extending slightly under the gate. In one embodiment, haloregions 448 are provided for the S/D regions. The halo regions aresecond polarity type doped regions which extend beyond the LD extensionregions.

In one embodiment, the first and second transistors share a common drainregion 445. The gates of the transistors serve as wordlines. In oneembodiment, the first gate of the first transistor 441 a serves as aWL_w and the second gate of the second transistor 441 b serves as aWL_r/w.

As shown, a well contact 409 is disposed in the well 408. The wellcontact is a second polarity type heavily doped region. For example, thewell contact is a heavily doped p-type region for an n-type device. Thewell contact provides a back bias voltage to the device well to bias thebody of the transistors of the selectors.

Disposed over the transistors is a dielectric layer 490. The dielectriclayer serves as a backend dielectric layer. The backend dielectric layermay include one or more interlevel dielectric (ILD) layers. An ILD layerincludes a metal level and a contact level. The metal level includesconductors or metal lines while the contact level includes contacts. Theconductors and contacts may be formed of a metal, such as copper, copperalloy, aluminum, tungsten or a combination thereof. Other suitable typesof metals, alloys or conductive materials may also be useful. In somecases, the conductors and contacts may be formed of the same material.For example, in upper metal levels, the conductors and contacts may beformed by dual damascene processes. This results in the conductors andcontacts having the same material. In some cases, the conductors andcontacts may have different materials. For example, in the case wherethe contacts and conductors are formed by single damascene processes,the materials of the conductors and contacts may be different. Othertechniques, such as reactive ion etch (RIE), may also be employed toform metal lines.

As discussed, the backend dielectric layer may include a plurality ofILD levels. For example, x number of ILD levels may be provided. A metallevel of an ILD level may be referred to as M_(i), where i is the i^(th)ILD level of x ILD levels. A contact level of an ILD level may bereferred to as V_(i−1), where i is the i^(th) ILD level of x ILD levels.As shown, the dielectric layer includes ILD levels up to M2. Thedielectric layer may include additional ILD levels (not shown). Thenumber of levels may depend on design requirements. For example, thedielectric layer may include 6 ILD levels. For example, the dielectriclayer may include up to M6. Other suitable number of ILD levels may alsobe useful.

For the first contact level, it may be referred to as a CA or apre-metal dielectric (PMD) level. The first metal level may be referredto as M1. For the first ILD level, contacts and metal lines may beformed by separate single damascene processes. Contacts in the PMD levelmay be tungsten contacts while conductors or metal lines in M1 may beformed of copper or copper alloy. For upper ILD levels, metal lines andcontacts may be formed by dual damascene techniques. Dual damasceneprocesses may employ copper or copper alloys for contacts and conductivelines. Other configurations or techniques for forming lines and contactsof the ILD levels may also be useful.

A dielectric liner may be disposed between ILD levels. The dielectricliner, for example, serves as an etch stop layer. The dielectric linermay be formed of a low k dielectric. For example, the dielectric linermay be nBLOK. Other types of dielectric materials for the dielectricliner may also be useful.

As shown, contacts 492 are disposed in the CA level of the backenddielectric layer. The contacts, for example, are in communication withthe drain and source regions 445 and 446 of the select transistors andthe well contact 409. Conductive lines 494 are disposed in the firstmetal or M1 level above the CA level. In one embodiment, SLs are coupledto the source region of the select transistors. For example, a first SLis coupled to the source region of the first select transistor and asecond SL is coupled to the source region of the second selecttransistor. The SLs are connected to the sources via contacts in the CAlevel. In addition, an interconnect pad 496 is disposed in M1 which iscoupled to the well contact 409 via a contact 492 in the CA level. Astorage interconnect pad 497 is also provided in M1 which is coupled tothe drain 445 of the transistors.

A storage element 420 may be disposed in the storage dielectric layerabove the interconnects connecting to the various transistors of theselect unit. For example, the storage elements may be disposed over thefirst metal level. The storage dielectric layer may be a via level V1,as shown above M1. Providing the storage dielectric layer in other upperlevels may also be useful. The storage element may be provided in astorage dielectric level.

The storage element, in one embodiment, is a MTJ element, as describedin FIGS. 1a-1b . Other types of storage or resistive storage elementsmay also be useful. For example, the MTJ element includes a tunnelingbarrier layer 427 disposed between fixed and free layers 426 and 428. Asshown, the MTJ element is a bottom pinned MTJ element. In oneembodiment, the MTJ element is a bottom pinned perpendicular MTJelement. The MTJ element may include top and bottom electrodes (notshown), such as Ta electrodes. Other types or configurations of thestorage elements may also be useful.

Above the storage dielectric layer is a metal layer of an ILD level. Forexample, the metal layer is M2. Other metal layers may also be useful.The metal layer includes metal lines disposed in a dielectric layer. Themetal line and top of the dielectric layer have coplanar top surfaces.In one embodiment, a bit line (BL) is coupled to the storage element.

Although as described, the various lines and storage element aredisposed in specified dielectric levels of the backend dielectriclevels, other configurations may also be useful. For example, they maybe disposed in other or additional metal levels. For example, thestorage element may be provided in an upper via level, such as betweenM5 and M6. In such case, the BL may be disposed in metal level M6. TheBL may also be disposed in other suitable metal level. Furthermore, thedevice may include other device regions and components.

Further, in other embodiments, the device may be modified to include afully depleted silicon-on-insulator (FDSOI) substrate. The SOI substrateincludes a surface crystalline layer separated from a bulk crystallineby a buried oxide (BOX) layer. In such case, the back bias may beprovided through the BOX layer to increase the write current or toreduce the read current. A body contact may be disposed through the BOXto connect to the N-type region of the bulk crystalline forback-biasing. For example, a forward back bias (i.e. positive voltagefor NFET) applied to the N-type region of the bulk crystalline increasesthe write current and a reverse back bias (i.e. negative voltage forNFET) applied to the N-type region of the bulk crystalline reduces theread current.

FIGS. 5a-5b show top and cross-sectional views of another embodiment ofa device 500. The device, as shown, includes a memory cell. The memorycell, for example, may be a non-volatile memory (NVM) memory cell. Thememory cell, in one embodiment, is a magnetoresistive NVM cell, such asa STT-MRAM cell. The memory cell, for example, is similar to thatdescribed in FIGS. 2a and 4a-4b . Common elements may not be describedor described in detail. In the interest of brevity, the description ofthe device 500 below primarily focuses on the difference(s) comparedwith the device 400 described and shown in FIGS. 4a -4 b.

As shown, the memory cell includes a cell selector unit with first andsecond selectors 441 a-441 b, such as first and second MOS transistors.In one embodiment, the MOS transistors are n-type MOS transistors. Othertypes of cell selectors may also be useful. The first selector 441 a,for example, may be a write selector and the second selector 441 b is aread/write selector. In one embodiment, the first transistor 441 a is aLVT transistor while the second transistor 441 b is a HVT or RVTtransistor. In one embodiment, the halo regions of the first and secondtransistors may be tuned to include different dopant concentrations toproduce the desired HVT or RVT and LVT transistors. For example, thehalo regions 549 of the second transistor includes a higher secondpolarity type dopant concentrations relative to the halo regions 548 ofthe first select transistor. For example, the halo regions 549 includesa dopant concentration of greater than 2E18 cm⁻³ to form a HVTtransistor or a dopant concentration of about 1-2E18 cm⁻³ to form a RVTtransistor while the halo regions 548 includes a dopant concentration ofless than 1E18 cm⁻³ to form a LVT transistor. The dopant concentration,for example, is in increasing order from LVT to RVT to HVT. The dopantconcentration values as presented are exemplary and may change dependingon, for example, technology node. As described, the different dopantconcentrations of the halo regions may be tuned to produce the desiredHVT or RVT and LVT transistors. In another embodiment, different surfacethreshold voltage Vt implant in the substrate may be tuned to form thedesired HVT or RVT and LVT transistors.

Further, as shown, the bodies of the transistors are not back biased.For example, no well contacts are provided for biasing the bodies of thetransistors. In some embodiments, a well contact may be provided, asshown in FIGS. 4a-4b , to bias the bodies of the transistors. In suchcase, the read current could be further reduced by reverse back-biasingthe bodies of the transistors while the write current could be furtherincreased by forward biasing the bodies of the transistors. In addition,although the first and second selectors are illustrated as n-typetransistors, it is understood that the first and second selectors may bep-type transistors.

FIGS. 6a-6m show cross-sectional views of an embodiment of a process 600for forming a device. The process includes forming a memory cell. Thememory cell, for example, may be a NVM memory cell. The memory cell, inone embodiment, is a magnetoresistive NVM cell, such as a STT-MRAM cell.The memory cell, for example, is the same or similar to that describedin FIGS. 4a-4b . Common elements may not be described or described indetail.

The cross-sectional views, for example, are along the bit linedirection. Although the cross-sectional views show one memory cell, itis understood that the device includes a plurality of memory cells of,for example, a memory array. In one embodiment, the process of formingthe STT-MRAM cell is highly compatible with CMOS logic process. Forexample, the STT-MRAM cell can be formed simultaneously with CMOS logicdevices (not shown) on the same substrate.

Referring to FIG. 6a , a substrate 605 is provided. The substrate, forexample, is a semiconductor substrate, such as a silicon substrate. Forexample, the substrate may be a lightly doped p-type substrate.Providing an intrinsic or other types of doped substrates, such assilicon germanium (SiGe), germanium (Ge), gallium arsenide (GaAs) or anyother suitable semiconductor materials, may also be useful. In someembodiments, the substrate may be a crystalline-on-insulator (COI)substrate. A COI substrate includes a surface crystalline layerseparated from a crystalline bulk by an insulator layer. The insulatorlayer, for example, may be formed of a dielectric insulating material.The insulator layer, for example, includes silicon oxide, which providesa buried oxide (BOX) layer. Other types of dielectric insulatingmaterials may also be useful. The COI substrate, for example, is asilicon-on-insulator (SOI) substrate. For example, the surface and bulkcrystalline layers are single crystalline silicon. Other types of COIsubstrates may also be useful. It is understood that the surface andbulk layers need not be formed of the same material.

In FIG. 6a , the substrate is processed to define a cell region in whicha memory cell is formed. The cell region may be part of an array region.For example, the array region may include a plurality of cell regions.The substrate may include other types of device regions, such as a logicregion. Other types of regions may also be provided.

Isolation regions 680 are formed in the substrate. In one embodiment,the isolation regions are shallow trench isolation (STI) regions. Othertypes of isolation regions may also be useful. The isolation regions areprovided to isolate device regions from other regions. The isolationregions may also isolate contact regions within a cell region. Isolationregions may be formed by, for example, etching trenches in the substrateand filling them with a dielectric material, such as silicon oxide. Aplanarization process, such as chemical mechanical polishing (CMP), isperformed to remove excess dielectric material, leaving, for example,STI regions isolating the device regions.

Referring to FIG. 6a , a doped well or device well 608 is formed. Thewell, for example, serves as body of select transistors of the memorycell. In one embodiment, second polarity type dopants are implanted intothe substrate to form the doped well. The second polarity type dopants,for example, include p-type dopants. In one embodiment, an implant maskmay be employed to implant the dopants to form the doped well. Theimplant mask, for example, is a patterned photoresist layer. The implantmask exposes regions of the substrate in which the second polarity wellsare formed. The device well may be lightly or intermediately doped withsecond polarity type dopants. For example, the device well may have adopant concentration of about 1E16 to 1E19/cm³. Other dopantconcentrations may also be useful.

As shown in FIG. 6b , gate layers are formed on the substrate. The gatelayers, in one embodiment, include a gate dielectric layer 652 and agate electrode layer 654 thereover. The gate dielectric layer, forexample, may be a silicon oxide layer formed by, thermal oxidation. Asfor the gate electrode layer, it may be a polysilicon layer formed bychemical vapor deposition (CVD). Other suitable types of gate layers,including high k dielectric and metal gate electrode layers, or othersuitable techniques for forming gate layers may also be useful.

Referring to FIG. 6c , the gate layers are patterned to form gates 644of the first and second select transistors 641 a-641 b. Patterning thegate layers may be achieved using mask and etch techniques. For example,a soft mask, such as photoresist may be formed over the gate electrodelayer. An exposure source may selectively expose the photoresist layerthrough a reticle containing the desired pattern. After selectivelyexposing the photoresist layer, it is developed to form openingscorresponding to location where the gate layers are to be removed. Toimprove lithographic resolution, an anti-reflective coating may be usedbelow the photoresist layer. The patterned mask layer is used to patternthe gate layers. For example, an anisotropic etch, such as reactive ionetch (RIE), is used to remove exposed portions of the gate layers. Othertypes of etch processes may also be useful. The etch transfers thepattern of the mask layer to the underlying gate layers. Patterning thegate layers forms gates of the select transistors. The gates, forexample, may be gate conductors along a first or word line direction. Agate conductor forms a common gate for a row of memory cells. The gateof the first transistor 641 a serves as a WL_w while the gate of thesecond transistor 641 b serves as a WL_r/w.

Referring to FIG. 6d , an implant is performed to form LD extensionregions 647. An implant mask (not shown) may be used to form the LDextension regions. To form the LD extension regions, first polarity typedopants are implanted into the substrate. The first polarity typedopants, for example, include n-type dopants. In one embodiment, theimplant forms LD extension regions in the substrate adjacent to thegates. For example, the LD extension regions extend slightly under thegates. The LD extension regions, for example, include dopantconcentration of about 1E18 to 5E19 cm⁻³. Other dopant concentration mayalso be useful. In one embodiment, the process 600 may also includeperforming an implant to form halo regions 648. The halo regions areformed by implanting second polarity type dopants into the substrate.The second polarity type dopants, for example, include p-type dopants.The same implant mask to form the LD extension regions may be used toform the halo regions. In one embodiment, the implant forms halo regionsin the substrate adjacent to the gates. For example, the halo regionsextend slightly under the gates and extend beyond the LD extensionregions. The halo regions, for example, have a depth slightly deeperthan the LD extension regions. In one embodiment, the halo regions 648,for example, include the same dopant concentration for the first andsecond select transistors 641 a-641 b. The halo regions, for example,include dopant concentration of about 5E17 to 5E18 cm⁻³. Other dopantconcentration may also be useful.

The process 600 continues to form sidewall spacers on sidewalls of thegates 644. To form the spacers, a dielectric spacer layer 659, such assilicon oxide, is formed on the substrate covering the gates as shown inFIG. 6e . The spacer layer may be formed by CVD. Other techniques forforming the spacer layer may also be useful. An anisotropic etch, suchas RIE, is performed to remove horizontal portions of the spacer layer,leaving spacers 649 to abut the sidewalls of the gates.

Referring to FIG. 6f , an implant is performed to form first and secondS/D regions 645 and 646. An implant mask (not shown) may be used to formthe first and second S/D regions in the substrate. In one embodiment,the implant forms heavily doped first polarity type S/D regions in thesubstrate adjacent to the gates. The first polarity type dopants, forexample, include n-type dopants. The implantation process to form thefirst and second S/D regions may be performed together while formingfirst polarity type S/D regions in other device regions (not shown) onthe same substrate. The first and second heavily doped S/D regions 645and 646, for example, have a depth deeper than the LD extension regionsand halo regions. The first and second S/D regions 645 and 646, forexample, include dopant concentration of about 5E19 to 1E21 cm⁻³. Otherdopant concentration may also be useful. The first and second selecttransistors 641 a and 641 b, as shown, include a common drain region 645formed in the substrate in between the gates.

In one embodiment, the process 600 also includes forming a well contact609 in the well 608. The well contact is a second polarity type heavilydoped region. The second polarity type dopants, for example, includep-type dopants. An implant is performed to form the well contact 609. Animplant mask (not shown) may be used to form the well contact in thesubstrate. In one embodiment, the implant forms heavily doped secondpolarity type well contact region in the substrate adjacent to theisolation regions. The implantation process to form the well contact maybe performed together while forming second polarity type S/D regions inother device regions (not shown) on the same substrate. The well contact609, for example, may have the same depth and dopant concentration asthe first and second S/D regions 645 and 646. Other dopant concentrationmay also be useful. The well contact provides a back bias voltage to thedevice well to bias the body of the transistors of the selectors.

A dielectric etch stop liner (not shown) may be formed over thetransistors 641 a-641 b. The etch stop liner, for example, is a nitrideetch stop liner. Other types of dielectric etch stop liners may also beuseful. The etch stop liner serves as an etch stop for subsequentprocesses, such as contacts formation.

Referring to FIG. 6g , a dielectric layer 690 ₁ is formed on thesubstrate, covering the transistors. The dielectric layer, for example,serves as a dielectric layer of an ILD layer. For example, thedielectric layer serves as a PMD or CA level of an ILD layer. Thedielectric layer, for example, is a silicon oxide layer. Other types ofdielectric layers may also be useful. The dielectric layer may be formedby CVD. Other techniques for forming the dielectric layer may also beuseful. A planarizing process may be performed to produce a planarsurface. The planarizing process, for example, may include CMP. Othertypes of planarizing processes may also be useful.

In one embodiment, contacts 692 are formed in the dielectric layer 690 ₁as shown in FIG. 6h . The contacts, for example, connect to contactregions, such as well contact, S/D regions and gates of the transistors.Forming the contacts may include forming contact vias in the dielectriclayer to expose the contact regions. As shown, the contacts 692 arecoupled to the well contact 609 and the S/D regions 645 and 646 of thetransistors. Forming the contact vias may be achieved using mask andetch techniques. After the vias are formed, a conductive material isdeposited to fill the vias. The conductive material, for example, may betungsten. Other types of conductive materials may also be useful. Aplanarization process, such as CMP, is performed to remove excessconductive material, leaving contact plugs in the contact vias.

In FIG. 6h , a dielectric layer 690 ₂ is formed over the substrate,covering the lower dielectric layer 690 ₁. The dielectric layer, forexample, serves as a metal level of an ILD layer. In one embodiment, thedielectric layer serves as M1 level of the ILD layer. The dielectriclayer, for example, is a silicon oxide layer. Other types of dielectriclayers may also be useful. The dielectric layer may be formed by CVD.Other techniques for forming the dielectric layer may also be useful.Since the underlying surface is already planar, a planarizing processmay not be needed. However, it is understood that a planarizationprocess, such as CMP, may be performed if desired to produce a planarsurface.

Conductive or metal lines 694 are formed in the dielectric layer 690 ₂.The conductive lines may be formed by damascene technique. For example,the upper dielectric layer may be etched to form trenches or openingsusing, for example, mask and etch techniques. A conductive layer isformed on the substrate, filling the openings. For example, a copper orcopper alloy layer may be formed to fill the openings. The conductivematerial may be formed by, for example, plating, such as electro orelectroless plating. Other types of conductive layers or formingtechniques may also be useful. In one embodiment, source lines SLs areformed to connect to the source regions 646 of the transistors whileother interconnects, such as interconnect pad 696 formed in M1 iscoupled to the well contact 609 and a storage interconnect pad 697formed in M1 is coupled to the common drain region 645 of thetransistors.

As shown in FIG. 6j , the process continues to form storage element 620of the memory cell. In one embodiment, the process forms an MTJ elementof the memory cells. For example, various layers of MTJ stack are formedon the dielectric layer and patterned to form the MTJ element of thememory cell. Forming the storage elements may include forming thevarious layers of the MTJ stack on the dielectric layer 690 ₂. Thelayers may include materials as described in FIGS. 1a-1b and may beformed by various deposition techniques. The deposition techniques maydepend on the type of layer. The layers may be patterned to form the MTJelement. Patterning the layers may be achieved using an anisotropicetch, such as RIE, with a patterned mask layer. Other techniques forforming the MTJ element may also be useful.

Referring to FIG. 6k , a storage dielectric layer 690 ₃ is formed overthe MTJ element. The dielectric layer covers the storage element 620.The storage dielectric layer, for example, is a silicon oxide layer. Thestorage dielectric layer may be formed by, for example, CVD. Other typesof storage dielectric layers or forming techniques may also be useful. Aplanarization process is performed to remove excess dielectric materialto form a planar surface. The planarization process, for example, isCMP. The planarization process exposes the top of the storage elementand provides a planar surface.

In FIG. 6l , a dielectric layer 690 ₄ is formed over the substrate,covering the storage dielectric layer. The dielectric layer, forexample, serves as a metal level of an ILD layer. In one embodiment, thedielectric layer serves as a metal level of the storage dielectriclevel. For example, the dielectric layer serves as M2 while the storagedielectric layer serves as V1. Other ILD levels may also be useful. Thedielectric layer, for example, is a silicon oxide layer. Other types ofdielectric layers may also be useful. The dielectric layer may be formedby CVD. Other techniques for forming the dielectric layer may also beuseful. Since the underlying surface is already planar, a planarizingprocess may not be needed. However, it is understood that aplanarization process, such as CMP, may be performed to produce a planarsurface.

As shown in FIG. 6m , a conductive or metal line is formed in thedielectric layer 690 ₄ and on the storage dielectric layer 690 ₃. Themetal line may be formed using, for example, damascene technique. Forexample, BL may be formed in the dielectric layer 690 ₄. This provides aconnection for the storage element to BL. For example, the MTJ element620 is coupled to the BL.

Additional processes may be performed to complete forming the device.For example, the processes may include forming additional ILD levels,pad level, passivation level, pad opening, dicing, assembly and testing.Other types of processes may also be performed.

Further, the substrate 605 as illustrated in process 600 is a bulksubstrate. In other embodiments, the process may be modified to providea fully depleted silicon-on-insulator (FDSOI) substrate. The SOIsubstrate includes a surface crystalline layer separated from a bulkcrystalline by a buried oxide (BOX) layer. In such case, the process maybe modified by performing a hybrid process to remove portions of thesurface crystalline and BOX layer so that a body contact can be formedto connect to the N-type region of the bulk crystalline forback-biasing.

FIGS. 7a-7d show cross-sectional views of another embodiment of aprocess 700 for forming a device. The process includes forming a memorycell. The memory cell, for example, is the same or similar to thatdescribed in FIGS. 5a-5b and the process 700 is similar to process 600as described in FIG. 6a-6m . Thus, common elements may not be describedor described in detail. In the interest of brevity, the description ofthe process 700 below primarily focuses on the difference(s) comparedwith the process 600 described and shown in FIGS. 6a -6 m.

As shown in FIG. 7a , a partially processed substrate is provided. Thesubstrate is processed at the stage similar to that described in FIG. 6c. For example, gates 644 of the first and second transistors 641 a-641 bare formed on the substrate.

Referring to FIG. 7b , an implant is performed to form LD extensionregions 647. The process parameters for forming the LD extension regionsare the same as that described in FIG. 6d . In one embodiment, theprocess 700 also forms halo regions in the substrate. In one embodiment,a first halo implant is performed by implanting second polarity typedopants into the substrate. The second polarity type dopants, forexample, include p-type dopants. The same implant mask (not shown) toform the LD extension regions may be used to form the halo regions. Inone embodiment, the implant forms halo regions 748 in the substrateadjacent to the gates of the first and second transistors. For example,the halo regions extend slightly under the gates and extend beyond theLD extension regions. The halo regions, for example, have a depthslightly deeper than the LD extension regions. In one embodiment, thefirst halo implant, for example, is performed at an angle of 15-45degrees with reference to the substrate surface and include a dopantconcentration of greater than 2E18 cm⁻³. Other suitable angles anddopant concentrations may also be useful. The implant mask is removed,for example, by ashing.

The process 700 continues by providing another implant mask 740. Theimplant mask, in one embodiment, covers a portion of the substrate wherethe first transistor 641 a is disposed, while a portion of the substratewhere the second transistor 641 b is disposed is exposed. For example,the implant mask 740 covers the gate 644 of the first transistor 641 aand portions of the LD extension regions of the first transistor 641 a,while the gate 644 of the second transistor 641 b and portions of the LDextension regions of the second transistor 641 b are exposed. A secondhalo implant is performed by implanting second polarity type dopantsinto exposed portions of the substrate. The second polarity typedopants, for example, include p-type dopants. In one embodiment, thesecond halo implant introduces second polarity type dopants into exposedportions of the substrate adjacent to the gate of the second transistor.In one embodiment, the second halo implant, for example, is performed atan angle of 15-45 degrees with reference to the substrate surface andinclude a dopant concentration of about 1E17 to 2E18 cm⁻³. Othersuitable angles and dopant concentrations may also be useful. Theimplant mask 740 is removed, for example, by ashing.

The dopant concentration in the halo regions affect the thresholdvoltage of the transistors. As shown, the halo regions 749 formed inexposed portions of the substrate adjacent to the gate of the secondtransistor 641 b include a dopant concentration that is introduced byboth the first and second halo implants and is higher than that of thehalo regions 748 formed in portions of the substrate adjacent to thegate of the first transistor 641 b. The combination of the first andsecond halo implants produces a high threshold voltage (HVT). Thus, thesecond transistor 641 b is a HVT transistor while the first transistor641 a which is subject to the first halo implant is a low thresholdvoltage (LVT) transistor.

As described, the different dopant concentrations of the halo regionsmay be tuned to produce the desired HVT or RVT and LVT transistors. Inanother embodiment, different surface threshold voltage Vt implant maybe tuned to form the desired HVT or RVT and LVT transistors.

The process continues with processing steps as described in FIG. 6e andonwards. For example, the process continues to form sidewall spacers,heavily doped S/D regions, forming dielectric layers, via contacts,metal lines, storage elements, etc. Additional processing is performedon the substrate until a device such as that shown in FIG. 7d is formed.Details of these processing steps are the same as that described inFIGS. 6e-6m and therefore will not be described or described in detail.

The embodiments as described result in various advantages. Theembodiments as described above improves read stability while achievingoptimal write robustness of the memory cell. For example, the memorycell with the reversed back biased improves read margin and two WLs areutilized during write operation provides for high write current. Inaddition, providing a low threshold for the write transistor and a highthreshold for the read transistor improves writing speed of the writetransistor. The processes as described are also highly compatible withCMOS logic processing and could be implemented using current technologyand equipment. Thus, no additional cost is required for implementing theprocesses as described above.

The present disclosure may be embodied in other specific forms withoutdeparting from the spirit or essential characteristics thereof. Theforegoing embodiments, therefore, are to be considered in all respectsillustrative rather than limiting the invention described herein. Scopeof the invention is thus indicated by the appended claims, rather thanby the foregoing description, and all changes that come within themeaning and range of equivalency of the claims are intended to beembraced therein.

What is claimed is:
 1. A memory cell comprising: a substrate definedwith a memory cell region; a cell selector unit disposed on thesubstrate, wherein the cell selector unit comprises a first selecttransistor having a first gate coupled to a first word line (WL) andfirst and second source/drain (S/D) regions, and a second selecttransistor having a second gate coupled to a second WL and first andsecond S/D regions, wherein the first select transistor serves as awrite selector and the second select transistor serves as a readselector, the first select transistor is configured as a low thresholdvoltage (LVT) transistor and the second select transistor is configuredas a high threshold voltage (HVT) or regular threshold voltage (RVT)transistor, the first WL is a write wordline (WL_w) and the second WL isa read/write wordline (WL_r/w), and the second S/D regions of the firstand second select transistors are coupled to a common source line (SL);a body well disposed in the substrate, wherein the body well serves as abody of the first and second select transistors, wherein a back bias isapplied to the body of the select transistors; and a storage elementwhich comprises a magnetic tunnel junction (MTJ) element coupled with abitline (BL) and the first and the second select transistors.
 2. Thememory cell of claim 1 wherein the S/D regions comprise first polaritytype dopants and the body well comprises second polarity type dopants,wherein the second polarity type is opposite to the first polarity type.3. The memory cell of claim 1 wherein the second S/D regions of thefirst and second select transistors are a common drain region.
 4. Thememory cell of claim 3 wherein the common drain region of the first andsecond select transistors are coupled to the MTJ element.
 5. The memorycell of claim 1 wherein the first and second select transistors comprisehalo regions disposed adjacent to first and second sides of the firstand second gates of the select transistors, the halo regions adjacent tothe second gate of the second select transistor comprise a higher dopantconcentration relative to the halo regions adjacent to the first gate ofthe first select transistor.
 6. The memory cell of claim 1 wherein thehalo regions adjacent to the first and second gates of the first andsecond select transistors are tuned to include different dopantconcentrations to produce the HVT or RVT and the LVT transistors.
 7. Amemory cell comprising: a substrate defined with a memory cell region; acell selector unit disposed on the substrate, wherein the cell selectorunit comprises a first select transistor having a first gate coupled toa first word line (WL) and first and second source/drain (S/D) regions,and a second select transistor having a second gate coupled to a secondWL and first and second S/D regions, wherein the first select transistorserves as a write selector and the second select transistor serves as aread selector, the first WL is a write wordline (WL₁₃w) and the secondWL is a read/write wordline (WL_r/w), and the second S/D regions of thefirst and second select transistors are coupled to a source line (SL); abody well disposed in the substrate, wherein the body well serves as abody of the first and second select transistors, wherein a back bias isapplied to the body of the select transistors; a storage element whichcomprises a magnetic tunnel junction (MTJ) element coupled with abitline (BL) and the first and the second select transistors; andwherein a forward back bias is applied to the body of the selecttransistors to increase a write current to write the MTJ element and areverse back bias is applied to the body of the select transistors toreduce a read current to read the MTJ element.
 8. The memory cell ofclaim 7 wherein the forward back bias comprises a positive voltagegreater than 0 to 0.6V.
 9. The memory cell of claim 7 wherein thereverse back bias comprises a negative voltage less than 0 to −2V.
 10. Amemory cell comprising: a substrate defined with a memory cell region; acell selector unit disposed on the substrate, wherein the cell selectorunit comprises a first select transistor having a first gate coupled toa first word line (WL) and first and second source/drain (S/D) regions,and a second select transistor having a second gate coupled to a secondWL and first and second S/D regions, wherein the first select transistorserves as a write selector and the second select transistor serves as aread selector, the first select transistor is a low threshold voltage(LVT) transistor and the second select transistor is a high thresholdvoltage (HVT) or regular threshold voltage (RVT) transistor, wherein thefirst and second select transistors comprise halo regions disposedadjacent to first and second sides of the first and second gates of theselect transistors, the S/D regions comprise first polarity type dopantsand the halo regions comprise second polarity type dopants, wherein thesecond polarity type is opposite to the first polarity type, the haloregions adjacent to the second gate of the second select transistorcomprise a higher second polarity type dopant concentration relative tothe halo regions adjacent to the first gate of the first selecttransistor, the first WL is a write wordline (WL_w) and the second WL isa read/write wordline (WL_r/w), and the second S/D regions of the firstand second select transistors are coupled to a common source line (SL);a storage element which comprises a magnetic tunnel junction (MTJ)element coupled with a bitline (BL) and the first and the second selecttransistors; and wherein the HVT or RVT transistor decreases a readcurrent during a read operation to improve stability.
 11. The memorycell of claim 10 wherein the halo regions adjacent to the first andsecond gates of the first and second select transistors are tuned toinclude different dopant concentrations to produce the HVT or RVT andthe LVT transistors.
 12. The memory cell of claim 10 wherein differentsurface threshold voltage Vt implant in the substrate are tuned to formthe HVT or RVT and LVT transistors.
 13. The memory cell of claim 10comprising a body well disposed in the substrate, wherein the body wellserves as a body of the first and second select transistors, wherein aback bias is applied to the body of the select transistors.
 14. Thememory cell of claim 10 wherein a forward back bias is applied to thebody of the select transistors to increase a write current to write theMTJ element and a reverse back bias is applied to the body of the selecttransistors to reduce a read current to read the MTJ element.
 15. Amethod of operating a memory cell comprising: providing a memory cellcomprising a substrate defined with a memory cell region, a cellselector unit disposed on the substrate, wherein the cell selector unitcomprises a first select transistor having a first gate coupled to afirst word line (WL) and first and second source/drain (S/D) regions,and a second select transistor having a second gate coupled to a secondWL and first and second S/D regions, wherein the first select transistorserves as a write selector and the second select transistor serves as aread selector, the first select transistor is configured as a lowthreshold voltage (LVT) transistor and the second select transistor isconfigured as a high threshold voltage (HVT) or regular thresholdvoltage (RVT) transistor, the first WL is a write wordline (WL_w) andthe second WL is a read/write wordline (W_r/w), and the second S/Dregions of the first and second select transistors are coupled to acommon source line (SL), a body well disposed in the substrate, whereinthe body well serves as a body of the first and second selecttransistors, wherein a back bias is applied to the body of the selecttransistors, and a storage element which comprises a magnetic tunneljunction (MTJ) element coupled with a bitline (BL) and the first and thesecond select transistors; and performing a read operation or writeoperation with the memory cell, wherein when a write operation isperformed, the WL_w and the WL_r/w are activated such that a write pathis formed through both the first and second select transistors, and whena read operation is performed, the WL_w is inactive and the WL_r/w isactivated such that a read path is formed through the second selecttransistor.
 16. The method of claim 15 wherein a forward back bias isapplied to the body of the select transistors to increase a writecurrent to write the MTJ element and a reverse back bias is applied tothe body of the select transistors to reduce a read current to read theMTJ element.
 17. A method of forming a memory cell comprising: providinga substrate defined with a memory cell region; forming a cell selectorunit on the substrate, wherein forming the cell selector unit comprisesforming a first select transistor having a first gate coupled to a firstword line (WL) and first and second source/drain (S/D) regions, andforming a second select transistor having a second gate coupled to asecond WL and first and second S/D regions, wherein the first selecttransistor serves as a write selector and the second select transistorserves as a read selector, the first WL is a write wordline (WL_w) andthe second WL is a read/write wordline (WL_r/w), and the second S/Dregions of the first and second select transistors are coupled to asource line (SL); forming a body well in the substrate, wherein the bodywell serves as a body of the first and second select transistors, andapplying a back bias to the body of the select transistors, whereinapplying the back bias comprises applying a forward back bias to thebody of the select transistors to increase a write current to write theMTJ element, and applying a reverse back bias to the body of the selecttransistors to reduce a read current to read the MTJ element; andforming a storage element which comprises a magnetic tunnel junction(MTJ) element coupled with a bitline (BL) and the first and the secondselect transistors.
 18. The method of claim 17 wherein the forward backbias comprises a positive voltage greater than 0 to 0.6V and the reverseback bias comprises a negative voltage less than 0 to −2V.
 19. Themethod of claim 17 wherein the first select transistor is configured asa low threshold voltage (LVT) transistor and the second selecttransistor is configured as a high threshold voltage (HVT) or regularthreshold voltage (RVT) transistor.
 20. The method of claim 17 whereinthe first and second select transistors comprise halo regions disposedadjacent to first and second sides of the first and second gates of theselect transistors, the halo regions adjacent to the second gate of thesecond select transistor comprise a higher dopant concentration relativeto the halo regions adjacent to the first gate of the first selecttransistor.